今天要來介紹很酷的 contenttypes,
官方文件contenttypes
Django includes a contenttypes application that can track all of the models installed in your Django-powered project, providing a high-level, generic interface for working with your models.
簡單說, 可以透過 contenttypes 連結全部的 model,
首先, 如果你沒有特別設定, 預設都是有啟用的,
就在 settings.py 底下的 INSTALLED_APPS,
INSTALLED_APPS = [
"django.contrib.admin",
"django.contrib.auth",
"django.contrib.contenttypes", # <<<<<<<
"django.contrib.sessions",
"django.contrib.messages",
"django.contrib.staticfiles",
"django_extensions",
"musics",
]先把 db 啟動, docker compose up -d,
然後 migrate
python3 manage.py makemigrations
python3 manage.py migrate其實 django_content_type 就是一張表, 等等下面會提到,
他負責紀錄和其他 model 之前的關係.
接著進入 shell
python3 manage.py shell開始介紹之前, 先看 models.py
from django.db import models
from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
from django.contrib.contenttypes.models import ContentType
# class Sheet(models.Model):
# name = models.CharField(default="sheet name")
# class Sheet2(models.Model):
# name = models.CharField(default="sheet2 name")
# class Music(models.Model):
# song = models.TextField(default="song")
# singer = models.TextField(default="AKB48")
# count = models.IntegerField(blank=True, null=True)
# last_modify_date = models.DateTimeField(auto_now=True)
# created = models.DateTimeField(auto_now_add=True)
# sheet = models.ForeignKey(Sheet, on_delete=models.CASCADE, null=True)
# sheet2 = models.ForeignKey(Sheet2, on_delete=models.CASCADE, null=True)
# class Meta:
# db_table = "music"
# def display_type_name(self):
# return self.get_type_display()
class Sheet(models.Model):
name = models.CharField(default="sheet name")
# 如果有加入這行, 可以反查(在 table 中不會增加欄位)
musics = GenericRelation('Music')
class Sheet2(models.Model):
name = models.CharField(default="sheet2 name")
class Music(models.Model):
song = models.TextField(default="song")
content_type = models.ForeignKey(
ContentType,
blank=True,
null=True,
on_delete=models.CASCADE,
)
object_id = models.PositiveIntegerField(blank=True, null=True)
# 在 table 中不會增加欄位
content_object = GenericForeignKey("content_type", "object_id")
class Meta:
db_table = "music"註解掉的部份是正常的寫法, 沒註解掉的是透過 contenttypes 的寫法,
仔細看兩個的差異, 你會發現不需要寫那個多的 ForeignKey 了,
多出來的是 content_type object_id content_object,
直接來看範例,
# 先建立一個 Sheet 和 一個 Sheet2 和一個 User
>>> from musics.models import Sheet, Sheet2
>>> sheet = Sheet.objects.create(name="sheet")
>>> sheet2 = Sheet2.objects.create(name="sheet2")
# 再建立一個內建的 User
from django.contrib.auth.models import User
user = User.objects.create(username="user1")# 建立 Music
>>> from musics.models import Music
>>> music_1 = Music.objects.create(song="song", content_object=sheet)
>>> music_2 = Music.objects.create(song="song", content_object=sheet2)
>>> music_3 = Music.objects.create(song="song", content_object=user)
>>> music_1, music_1.content_type, music_1.content_type.name
(<Music: Music object (1)>, <ContentType: musics | sheet>, 'sheet')
>>> music_2, music_2.content_type, music_2.content_type.name
(<Music: Music object (2)>, <ContentType: musics | sheet2>, 'sheet2')
>>> music_3, music_3.content_type, music_3.content_type.name
(<Music: Music object (3)>, <ContentType: auth | user>, 'user')你會發現, content_object 可以放入任何的物件,
(這邊放了 Sheet, Sheet2, User)
是不是非常的強大!!
如果你去看資料庫, music table 如下,
postgres=# SELECT id, song, content_type_id, object_id FROM public.music;
postgres-#
id | song | content_type_id | object_id
----+------+-----------------+-----------
1 | song | 7 | 1
2 | song | 9 | 1
3 | song | 4 | 1
(3 rows)object_id 對應的是 model 中的 id,
content_type_id 對應的是 model 中的關係 (就是 django_content_type table),
django_content_type table 如下,
postgres=# SELECT * FROM public.django_content_type;
id | app_label | model
----+--------------+-------------
1 | admin | logentry
2 | auth | permission
3 | auth | group
4 | auth | user
5 | contenttypes | contenttype
6 | sessions | session
7 | musics | sheet
8 | musics | music
9 | musics | sheet2
(9 rows)7 sheet model
9 sheet2 model
4 user model
其實他的原理就是透過 django_content_type 這張表去幫你找出答案的.
如果今天你想從 Sheet 去反查出 Music 也是可以,
在 Sheet 底下我們有加入 musics = GenericRelation('Music')
>>> from musics.models import Sheet, Sheet2
sheet = Sheet.objects.get(pk=1)
>>> sheet.musics.all() # 成功反查, 因為有設定 GenericRelation
<QuerySet [<Music: Music object (1)>]>
sheet2 = Sheet2.objects.get(pk=1)
>>> sheet2.musics.all() # 反查失敗, 因為沒設定 GenericRelation
Traceback (most recent call last):
File "<console>", line 1, in <module>
AttributeError: 'Sheet2' object has no attribute 'musics'- Python 3.9
文章都是我自己研究內化後原創，如果有幫助到您，也想鼓勵我的話，歡迎請我喝一杯咖啡:laughing:
MIT license